"The United Nations, and in particular I, have been making it consistently clear that providing arms to either side would not address this current situation," Ban told reporters during a briefing. "There is no such military solution."

Ban said the only solution that can solve the conflict "sustainably" is a political one that should be sought through a hoped-for U.S.-Russian brokered peace conference in Geneva.

"The military path points directly towards the further disintegration of the country, destabilization of the region and inflammation of religious and communal tensions," Ban said.

While calling for the on-the-ground investigation to continue, Ban also cast some doubt over the United States' claim that the regime of Bashar Assad had used chemical weapons against rebel forces. He said:

"... the validity of any information on the alleged use of chemical weapons cannot be ensured without convincing evidence of the chain-of-custody. That is why I continue to emphasize the need for an investigation on the ground in Syria that can collect its own samples and establish the facts. Our goal remains a fully independent and impartial inquiry. I have complete faith in the integrity and professionalism of Dr. Sellström and his team.

"The use of chemical weapons by any party would be a crime against humanity. Given the seriousness of the allegations and the potentially grave consequences, I again urge the Syrian Government to grant Dr. Sellström's team the access we have long sought."

Update at 1:19 p.m. ET. White House On Intelligence:

During a press briefing, Deputy National Security Adviser Ben Rhodes explained that the United States had a preliminary intelligence assessment in April that pointed toward chemical weapons use by the regime, but it did not have "a high confidence level."

Over the intervening months that assessment firmed up. He said the U.S. pieced "reporting from individuals who were there on the ground," "physiological samples," "open source reporting from social media and other places" and "our own intelligence reporting" to increase its confidence level.

Rhodes said the use of chemical weapons violates "clear international norms."

"This should be a red line to the international community generally," he said.